Highly efficient, low power particle collection devices have been demonstrated using multiple electrode corona discharge arrays. The advantages of multiple electrode corona discharge arrays for particle collection are described in “System and Method for Spatially Selective Particulate Deposition And Enhanced Particulate Deposition Efficiency”, filed Apr. 18, 2006, having an application Ser. No. 11/405,787, issued as U.S. Pat. No. 7,261,764, and in “Corona Charging Device and Methods”, filed Mar. 11, 2003 having an application Ser. No. 10/386,252, issued as U.S. Pat No. 7,130,178, and in “Method And Apparatus for Concentrated Airborne Particle Collection”, filed Jun. 24, 2003, having an application Ser. No. 10/603,119 issued as U.S. Pat. No. 7,062,982 all of which are herein incorporated by reference.
A key circuit element needed for the proper operation of multiple electrode corona discharge arrays is a resistor electrically connected in series between the high voltage DC power supply and each corona electrode. This resistor is known as a ballast resistor. The main function of the ballast resistor is to limit the current through any individual corona electrode when the plasma is initiated and while operating at steady state.
The voltage at which an electrical discharge is initiated is known to vary for each corona electrode in a multiple electrode system. Furthermore, the resistance of the air following the initial electrical discharge lowers dramatically such that the voltage needed to sustain the discharge is significantly lower than the initial breakdown voltage. Given these factors, it is therefore possible to deliver all electrical power to the corona discharge through a single or small number of electrodes. The resulting non-uniform plasma would defeat the primary benefits of a multiple electrode corona discharge system; that is, uniformity of electric field and charge density in the particle collection zone.
Providing a ballast resistor for each corona electrode solves the plasma non-uniformity problem by limiting the power delivered to any single corona electrode. Power through a single electrode is limited by lowering the electrode voltage as more current passes through the ballast resistor to the electrode. The ballasting effect allows the power supply voltage to adjust to a voltage where other electrodes will initiate and sustain continuous plasma.
This ballasting function places a number of electrical requirements onto the ballast resistor. The two key requirements are voltage breakdown between the resistor terminals and the resistance value. These requirements vary with electrode geometry and plasma power density. The value for the voltage breakdown of the ballast resistor used for the electrostatic radial geometry particle concentrator at is typically 9 kV. The resistance value for each of the ballast resistor used for this concentrator is 2 Gohm.
Resistors having the above characteristics are produced commercially. However, the breakdown and resistance values are not usually in high demand for most electrical applications. As a result, these resistors are typically much more expensive than lower voltage, lower value resistors. As an example, a 50V, 100 kohm resistor in a surface mount package can usually be purchased for less than $0.01. The 10 KV, 1 Gohm resistors used in the radial collector are purchased in small quantities for about $1.00. For most commercial and industrial particle collection applications, the number of electrodes needed is typically greater than thirty and less than five hundred. The cost the plastic material needed to produce an equivalent of 108 1 Gohm, 10 kV resistors is about $0.50 yielding a 216× improvement in cost.
Thus, there remains a need in the art for a highly-efficient, geometrically flexible and cost-effective material that provides for the resistive ballasting of multi-corona discharge arrays.